1. Field of the Invention
The present invention generally relates to a mechanism for a portable chair which can self fold up and absorb impact as the seat is folded up.
2. The Prior Arts
A conventional portable chair usually includes a set of joining hinges mounted between the seat frame and the front legs at the left and right sides of the seat frame, which allows folding and unfolding of the seat. For collapsing the chair, a user can manually pivot the seat to a folded state lying substantially parallel to the legs of the chair. Because the manual folding may not be convenient to operate, certain portable chairs may be provided with a self-folding mechanism, as shown in FIG. 8. The self-folding mechanism has hanging holes A1 and B1 respectively through a mount bracket A on the seat frame and a restricting flange B on the rear leg, and engaging first and second hook elements C1 and C2 of a tension spring C with the hanging holes A1 and B1. When the seat is unfolded, the tension spring C is stretched to accumulate a spring force. When the seat is released, the spring force makes the seat be folded into a collapsed state.
The conventional tension spring C includes coils and the two hook elements C1, C2 extended from the two ends of the coils. Each of the hook elements is extended from a central axis of the tension spring C as shown in FIG. 8, and has a curved shape. The first and second hook elements C1 and C2 are aligned in a straight line. Moreover, the length of the spring is usually shorter than the space in which the spring is assembled. Accordingly, the spring has to be substantially stretched to engage its two hook elements with the corresponding hanging holes, and therefore the spring is always in a loaded state even when the seat is collapsed. As a result, when the seat is collapsed from the unfolded state, the resilient force loaded in the spring (including the initial spring stretching force when the spring is assembled and the resilient force accumulated when the seat is unfolded) is substantially greater than necessary, which may cause fatigue and permanent deformation of the spring. Because the deformed spring has a greater length, the hook elements C1 and C2 may easily disengage from the hanging holes A1 and B1, causing failure of the self-folding mechanism.
Moreover, the excessive spring force applied on the seat may cause substantial noise and vibration when the steel-made seat frame collides against the rear legs.